1. Field of the Invention
The present invention relates to an electric motor and a production method thereof, the electric motor having a rotor structure for avoiding a defect, such as bending of the rotor and breakage of a magnet, due to thermal strains of a magnetic steel plate, a shaft and the magnet, etc., which constitute the rotor.
2. Description of the Related Art
Generally, major components for constituting a rotor of a synchronous electric motor are a magnetic steel plate, a carbon steel shaft and a permanent magnet. The structure of the rotor may include: an SPM (Surface Permanent Magnet) structure wherein a permanent magnet is attached to a rotor surface; and an IPM (Interior Permanent Magnet) structure wherein a permanent magnet is embedded into a magnet electric plate.
In the prior art, an electric motor is required to have a large maximum output or maximum torque, while having a small inertia moment. Therefore, when designing or manufacturing an electric motor, a rotor structure thereof is usually configured to extend in the axial direction. In such an electric motor, a neodymium magnet (NdFeB) having high magnetic force is used in many cases. As a means for fixing the neodymium magnet to a core structure wherein magnetic steel plates are stacked, adhesive agent is usually used. As an adhesive agent, a one-component thermoset adhesive agent is widely used.
In order to avoid strain on a rotor having a plurality of magnets, some techniques have been proposed. For example, in Japanese Unexamined Patent Publication (Kokai) No. 2002-78257 discloses a rotor having a permanent magnet, and describes that “a motor of the invention has a plurality of poles and a rotor, wherein a gap is formed between neighboring permanent magnets of the poles. Even when the temperature is varied, thermal strain between the permanent magnet and the rotor is reduced, whereby detachment of the component may be limited.” Further, it is described that “in the motor of the invention, the gap between neighboring permanent magnets is filled with an adhesive agent and beads. By filling the gap between permanent magnets with the adhesive agent and beads having predetermined dimensions, the thickness between the neighboring permanent magnets may be constant, whereby the dimension of the motor may be easily controlled.”
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2011-205781 discloses an electric motor having a permanent magnet, and describes that “when a permanent magnet is inserted into a magnet insert space formed in a rotor core, thin rubbers are temporarily attached to upper and lower ends in relation to the inserting direction of the magnet, and then the permanent magnets are sequentially inserted. After one or more magnet is inserted so as to stack the magnets in the rotational direction, the rotor core is tightened from both ends thereof so that pressure is applied to the thin rubber on the end of the magnet, whereby a rotor structure is constituted.”
In the rotor of the prior art wherein a neodymium magnet is bonded and fixed to a rotor core having stacked magnet steel plate, strain is generated due to the difference in linear expansion coefficients of the neodymium magnet and iron which constitutes the rotor. After the two components having different linear expansion coefficients are bonded by an impregnating agent (or adhesive agent), as the temperature is varied from when the adhesive agent is cured, a difference occurs between the axial lengths of the iron and the magnet, whereby strain is generated. Due to strain, following defects may be generated.                The adhesive agent for adhering the magnet is cracked, and the magnet may detach.        The rotor may bend due to unevenness of the adhesive.        The magnet may cracked.        A gap may form between the stacked magnetic steel plate.        
In particular, in a rotor which is relatively long in the axial direction, a plurality of magnets are stacked in the axial direction, the above defects may seriously occur corresponding to the axial length thereof.
FIG. 11 concretely explains the above problems in a rotor of the prior art. A rotor 50 as shown in FIG. 11 is an IPM rotor, and has a rotor core 54 constituted by stacking annular magnetic steel plate 52, a plurality of permanent magnets 56 embedded into rotor core 54, and two end plates 58 which clamps magnetic steel plate 52 from both sides in the axial direction. Each permanent magnet 56 is a neodymium magnet which has a generally plate shape, and the shapes of the permanent magnets are identical to each other. The permanent magnets are constituted as a plate extending in the axial direction of the rotor, and the plate is inserted into a slot 60, extending in the axial direction, formed in rotor core 54. A major surface of each permanent magnet 56 is bonded to an inner surface of slot 60 by means of thermoset adhesive agent (or impregnating agent) 62.
From the state of section (a) of FIG. 11, when the temperature of rotor core 54 (at least a portion of rotor 54 near adhesive agent 62) reaches a curing temperature of adhesive agent 62, magnetic steel plate 52 having a relatively high linear expansion coefficient (for example, 12×10−6/K) expands in the generally axial direction of rotor 54 (in the horizontal direction), as shown in section (b) of FIG. 11. On the other hand, the axial size of permanent magnet 56 is not substantially changed, since a linear expansion coefficient of permanent magnet 56 in the axial direction is considerably small (in the case of the neodymium magnet, −0.5×10−6/K). In such a state, adhesive agent 62 is cured so that permanent magnet 56 and magnetic steel plate 52 are bonded.
Next, when the temperature of rotor core 54 is lowered to a lower limit of the operating temperature of the electric motor, magnetic steel plate 52 is contracts in the generally axial direction. In this regard, since magnet steel plate 52 is bonded and fixed to permanent magnet 56, a portion of magnetic steel plate 52 near permanent magnet 56 does not contract and a portion of magnetic steel plate 52 near the outer surface or the center of rotor core 54 contracts as the temperature decreases. As a result, a strain is generated and a stress resides in rotor core 54, as shown in section (c) of FIG. 11. Further, as shown in section (d) of FIG. 11, due to the stress by the strain, a crack 64 may occur in magnetic steel plate 52, the rotor shaft may bend (not shown), and/or a crack or damage may occur in permanent magnet 56.
In the technique of Japanese Unexamined Patent Publication (Kokai) No. 2002-78257, a gap is formed between the permanent magnets so that the stress by the strain due to the row of the magnets is not generated. However, when such technique is applied to the IPM structure, it is difficult to form a constant gap between the magnets, in terms of the structure or manufacturing thereof. On the other hand, the technique is applied to the SPM structure, it is necessary to position the permanent magnets while arranging a jig, etc., for forming a constant gap between the magnets, and thus there is a difficulty in manufacturing the structure.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2002-78257 describes that adhesive agent is filled in the gap between the magnets and polyethylene beads or silicone rubber beads are added to the adhesive agent. However, it is difficult to assuredly fill the adhesive agent in the gap between the magnets since such an operation depends on the characteristic of the adhesive agent and the skill of an operator. Further, in the operation, it is necessary to remove the adhesive agent which protrudes from the gap to the surface of the magnet. Moreover, when the beads are added to the adhesive agent, the characteristic of the adhesive agent may be affected. Since the distribution density of the beads may be uneven in the adhesive agent, adhesive effect of the adhesive agent may be unstable.
On the other hand, in the structure of Japanese Unexamined Patent Publication (Kokai) No. 2011-205781, an elastic thin plate is attached to a surface of the magnet facing in the axial direction, and fastening force is applied to the structure from both axial ends thereof, by means of fastening plates, so that pressing force is applied to the elastic thin plate. However, in the structure of Japanese Unexamined Patent Publication (Kokai) No. 2011-205781, adhesives are not used to fix the permanent magnets, and thus the structure is not intended to accommodate the strain due to the difference in the linear expansion coefficients. Further, the object of the invention of Japanese Unexamined Patent Publication (Kokai) No. 2011-205781 is to prevent the magnet from being damaged when the magnet is inserted and fixed in the manufacturing process of the rotor, and thus does not solve the defect due to the thermal strain.